John Negroni's theory — that every film made by Pixar occurs in the same universe and connected to every other Pixar film — relies on the premise that Boo from Monsters Inc. is actually a time-traveling witch from Brave. His hypothesis is knotty at times, and requires that you believe in far-fetched, perhaps loopy ideas—and it's exactly the kind of theory that thrives on the Internet.
You really should go read Negroni's masterwork, but the basic premise is that animals who could talk like those in A Bug's Life and Finding Nemo and sentient robots like Buzz Lightyear were set into motion by the blue goo that Merida (from Brave) discovered and turned her mom into a bear (a snapshot of the timeline Negroni created appears on the right) and result in human-less world populated by bugs. Along with his theory, Negroni includes some mind-shattering revelations, such as Andy from Toy Story 3 receiving a post card from Elle and Carl in Up (photo by Buzzfeed Rewind):
And that would make it seem that Andy is related to Carl and Ellie, and ... and... sorry, I'm just picking the pieces of my brain matter off the floor. "Half of me believes they totally did this from the get-go since they are more than capable of doing something so amazing," Negroni told Entertainment Weekly. "While the other half of me realizes that this is my love for Pixar movies manifesting itself in over-the-top conspiracy theories."